New radio but rear roof speakers don't work

I just installed an aftermarket radio to replace the OEM radio on a 2 door 98 tahoe. I've had problems with the old radio in that the rear speakers would only put out low volume. I thought it was the radio but now it looks like it's not that. I've read some things about hooking up the antenna wire etc. but all the wires in the new radio adaptor is used. Does this have anything to do to with an amp that I read about for the 4 door models.

ditch the rear roof speakers and just upgrade the door speakers..